Robots are automated devices able to manipulate objects using a series of links, which in turn are interconnected via one or more robotic joints. Each joint in a typical robot represents at least one independent control variable, i.e., a degree of freedom (DOF). End-effectors such as hands, fingers, or thumbs are ultimately actuated to perform a task at hand, e.g., grasping a work tool or an object. Therefore, precise motion control of the robot may be organized by the level of task specification, including object, end-effector and joint-level control. Collectively, the various control levels achieve the required robotic mobility, dexterity, and work task-related functionality.
Tendon transmission systems in particular are often used in robotic systems having relatively high DOF robotic hands, largely due to limited packaging space. Since tendons can only transmit forces in tension, i.e., in pull-pull arrangements, the number of actuators must exceed the DOF to achieve fully determined control of a given robotic finger. The finger needs only one tendon more than the number of DOF, known as an n+1 arrangement. If arranged correctly, the n+1 tendons can independently control the n DOF while always maintaining positive tensions. In this sense, an n DOF finger with only n tendons is underactuated, and the finger posture is underdetermined. This situation creates a null-space within which the finger posture is uncontrolled. In other words, the finger cannot hold a desired position and will flop in the null-space. However, having a reduced number of actuators can be an advantage. Space or power limitations can be significant in high DOF robotic hands. Each extra actuator and tendon transmission system greatly increases the demand on space and maintenance requirements.